Structural role of Nd2O3 as a dopant material in modified borate glasses and glass ceramics

Glasses in the system xNd2O3–(46 − x)B2O3–27CaO–24.4Na2O–2.6 P2O5 (0 ≤ x ≤ 4 mol%) have been prepared via conventional melt quenching technique. X-ray diffraction spectra have showed that the amorphous structure is dominant in glasses of Nd2O3 concentrations ≤ 0.5 mol%. But formation of a more ordered structure is confirmed at higher Nd2O3 values. Result based on differential scanning calorimetry shows an increase in glass transition temperature (Tg) with increasing Nd2O3 at expense of B2O3. The measured density is found to be increased whereas the molar volume is decreased with increasing Nd2O3 content. The calculated molar volume (Vm) and free spaces (Vf) are both decreased due to filling process which is suggested to be carried out by Nd3+ of larger size than that of B3+. Decreasing of Vm and Vf can reflect the increase in bridging bonds in the glass network which in turns results in increasing of Tg of the investigated compositions. The thermal heat treatment process has no effect on state of crystal formation in glass containing 4 mol% Nd2O3.


Introduction
A family of rare-earth (RE) elements is considered between the elements of the lanthanide group which shows pronounced biological activity as they are capable of replacing Ca 2? in investigated glass matrix [1,2]. Most of the recent studies have concentrated on enhancing the bioactivity of glass materials by the effect of thermal heat treatment processes or adding RE ions in lowest concentrations [3][4][5][6][7][8]. These advances include the preparation of special glass materials, or the introduction of certain useful elements in the matrix of the simple glass materials [9].
The research interest in the branch of borate glass systems is due to their anomalous behavior. Borate glass has been studied most recently in contrast with other traditional glasses due to its good optical and mechanical properties [8,10]. Borate glass systems are well-known to have a stable structure against corrosion processes. Borate oxide glasses are one of the most suitable for doping with RE 3? ions among the potential host glasses. [8][9][10][11][12][13][14].
Due to its high transparently, low melting point, high thermal stability, different coordination numbers and strong solubility of rare-earth ions, borate glass is an especially suitable optical medium [15,16]. In addition, RE-doped alkali borate glasses are interesting for the study of the effects of alkali ions, especially around rare-earth ions, on the glass forming network. Depending on the type and concentration of the modifier oxide, the addition of an alkali oxide has a strong effect on boron coordination and structural groups [13,17,18].
In the present work, the properties and structure of some glasses and glass ceramics containing Nd 2 O 3 have been investigated in terms of its promising features for biomedical applications due to the electronic configuration of Nd 2 O 3 . Although studies of such glasses have continuously been increasing, they still have been poorly studied as biomaterials. Most current research focuses on improvement of the bioactivity of the base glass materials. These improvements include incorporation of specific concentrations from Nd 2 O 3 as useful type of rear earth oxide, through the basic glass materials.
It was known that boron oxide is considered as an ideal strong amorphous forming material. This is because B 2 O 3 cannot be crystallized at normal circumstances or at ambient pressure. The crystallization cannot be occurred because of completion between several borate units or phases which eventually induce the amorphization of the system. In this paper, we try to solve the problem of amorphous substituting B 2 O 3 with Nd 2 O 3 which can play a role of agent for crystallization. Transformation of amorphous to crystalline phases reduces the release of boron ions and increases the material hardness.

Preparation of glasses
In the present work, glasses in the system xNd 2 O 3 -(46 -x)B 2 O 3 -27CaO-24.4Na 2 O-2.6P 2 O 5 , (0 B x B 4 mol%) have been prepared via conventional melt quenching method. Materials were obtained from mixtures of reagent grade CaCO 3 , Na 2 CO 3 , H 3 BO 3 , Nd 2 O 3 and (NH 4 ) 2 HPO 4 thoroughly mixed and placed in a Pt-Au crucible. The batches were first heat treated from room temperature to 600°C with a slow heating rate of 2°/min to remove NH 3 and H 2 O and were then melted at 1000-1200°C during 20-30 min before being quenched by pouring the melt between two metallic plates. Melting time and temperatures were optimized to limit P 2 O 5 volatilization and maintain the overall glass weight losses under 2%.

Measurements
X-Ray diffraction measurements are carried out with Shimadzu X-ray diffract meter (Dx-30, Metallurgy institute, El Tebbin-Cairo). The peak position and intensity values used to identify the type of material were compared with patterns in the international powder diffraction file (PDF) database complied by joint committee for powder diffraction standards (JCPDS).
By applying Archimedes principle, the densities of the prepared samples were measured with benzene as the immersion liquid. The density was calculated using the formula: where, W a is the weight in air, W b is the weight in benzene, and q b is the density of benzene.
The molar volumes (V m ) were calculated using the equation that follows: in which V m is molar volume (cm 3 /mol), M T is the molecular weight of the glass sample (g/mol) and q is the density (g/cm 3 ) of the sample. The DSC thermograms obtained for samples with different concentrations of Nd 2 O 3 . All samples were heated from ambient temperature to 1000°C at a heating rate of 10°C/min under argon atmosphere.

XRD spectroscopy
The glasses of low Nd 2 O 3 (x = 0 and 0.5 mol%). were colorless, transparent and amorphous in nature. The addition of Nd 2 O 3 at expense of B 2 O 3 has been shown to be very effective in enhancing the crystallization of the glasses. Precipitation of the small sized crystals in the main glass network lowers the sample transparency and the glass in such a case is partially devitrified.
The XRD spectra revealed that an amorphous structure is the characteristic feature of the samples containing B 0.5 mol% Nd 2 O 3 . Transformation into a crystalline structure occurred after introducing higher Nd 2 O 3 concentrations. It is seen from Fig. 1a, b that the XRD spectrium contains two broad diffraction bands located between 20-30°and 40-55°. Presence of these broaden bands reflects the amorphous structure of the glasses of low Nd 2 O 3 concentration (0 and 0.5 mol%). On the other hand, the glasses of higher Nd 2 O 3 concentrations become relatively opaque due to formation or precipitation of some crystalline species. Presence of sharp and intense diffraction line plectra in glasses of more Nd 2 O 3 content Fig. 1c-e can be correlated to formation of the more ordered crystalline phases in the matrix of the investigated materials. Comparisons between X-ray sharp diffraction line spectra of the studied materials with that of Ca 3 (-PO 4 Figure 2 represents the change of the determined crystallinity with increasing Nd 2 O 3 concentration. Then from the Figs. 1 and 2 one may expect that the crystallization process is offered mainly by effect of Nd 2 O 3 , since the material containing even limited addition of Nd 2 O 3 ([ 0.5 mol%) is crystallized. The number of diffraction lines in glasses of 1, 3 and 4 mol% Nd 2 O 3 is the same but change in intensities is the most observed parameter. This means that the types of the well-formed crystalline phases are the same but the content of the separated phases increases with increasing Nd 2 O 3 concentrations. These modifications are summarized in Fig. 2, which reflects a change in the structure throughout changes of crystallinity in glass. It can be seen from this figure that with increasing Nd 2 O 3 concentration, the crystallinity increases to reach its saturated values in the region between 3 and 4 mol%. This interpretation may account on the presence of several diffraction lines in spectra of glasses modified by more than 0.5 mol% Nd 2 O 3 (spectra c, d, e). Some of the wellformed crystalline phases, such as crystalline apatite (calcium phosphate crystals), are categorized as bioactive phases that are useful for the material to be used in the field of biodental and bioactive use [19][20][21][22][23][24][25][26].
There are two parameters that can play a role in improving the process of crystallization in the glasses Fig. 2 The changes of crystallization process depending on the concentration of Nd 2 O 3 being tested. The first is the replacement of B 2 O 3 with Nd 2 O 3 , as mentioned above. Secondly, the thermal heat treatment process (THT) is alternatively applied also to improve the crystallization behavior. The latter can be applied on sample containing 4 mol% Nd 2 O 3 which characterized with its higher crystallinity in comparison with composition of lower Nd 2 O 3 . The temperature at which THT process can be considered can be extracted from DSC curves.

DSC, thermal treatments, glass transition temperature and Vicker hardness
The maximum crystallinity is found in composition of 4 mol% Nd 2 O 3 under the effect of glass composition. To assure the stability of the well-formed crystals, the sample of 4 mol% Nd 2 O 3 is also investigated under the effect of thermal treated at a specific temperature based on differential scanning ceilometer (DSC) data. It can be seen from Fig. 3, the DSC curve clearly shows one endothermic peak and one exothermic peak for all composition.. The endothermic peak corresponds to the glass transition (T g ) while the exothermic peak indicates the crystallization point of the glass (T c ). The glass transition temperature (T g ) as well as crystallization temperature (T c ) are estimated by the slope intercept method. The nature of the DSC curves is typical for other glass compositions. The crystallization temperature was found to be around 800°C. The glass is therefore thermally treated at this temperature. It can be seen from Fig. 4 that the state of crystallization did not changes with thermal heat treatment, since XRD spectra of the as prepared and treated samples are nearly not differed. This means that the glass ceramic of 4 mol% Nd 2 O 3 is the most recommended composition containing the maximum concentration of crystals.
The thermal analysis of the glasses was carried out because any change in the coordinating number of atom-forming networks or in the formation of nonbridging oxygen (NBO) or bridging bonds (BB) can simply be expressed by the change of T g with the composition. The variation of T g with compositions is shown in Fig. 5. It can be seen that with the rise of Nd 2 O 3 content, which is the network intermediate here, T g increases monotonically. It is documented that with the increase of bridging bonds in the main borate glass network, T g and crystallization temperature T c are generally increased [20,[27][28][29][30]. It is believed that T g depends on the strength of chemical bonds in the structure. Nd 2 O 3 in general, plays the role of a network intermediate which has been consumed to increase the bonds between different structural units with the increase of its content in the glass system. Increase of bridging oxygen indicates the increase in the strength of chemical bonds, which in turn increases T g as shown in Fig. 5.

Density, molar volume, free volume and packing density
It was found that the density of glass samples increases with increasing Nd 2 O 3 concentration as shown in Fig. 6. This is due to the higher molecular weight of Nd 2 O 3 (336.4822 g/mol) than the host structures of the glass samples (B 2 O 3 is 69.6202 g/mol) [19][20][21][22]. Therefore, the molar volume (M v ) shows a reverse behavior to density as shown in Fig. 6. The M v is the parameter that describes the volume occupied by the unit mass of a glass plus the free volume (V f ) surrounded the structural unit forming the network of the glass. In general, the unit mass is increased upon increasing Nd 2 O 3 at the expense of B 2 O 3 . In addition, the free spaces (V f ) associated with borate or NdO 4 units is decreased as a result of its occupation with Nd 3? ions which is of larger size than that of B 3? . Then, substitution of B 2 O 3 with Nd 2 O 3 is therefore decreases the free volume with a manner which depends on the ionic radius of the glass modifier oxide [23]. As a result, increase of density and decrease of free volumes (V f ) are the two factors played the role of decreasing (M v ) of the investigated glasses. The decrease of the molar volume is due to adding Nd 3? of larger ionic radius (1.123 Å ) into interstitial of host structure as the ionic radius of B 3 is (0.400 Å ) lead to reducing the free spaces formed around the structural units. means that the packing density should be increased with decreasing the total molar volume of the glass samples [22][23][24]. Then increasing P d (density) and decreasing void spaces in the glass network are considered as the main causes in increasing T g which leads to increasing the hardness of the investigating samples (Fig. 7).

Data availability
As authors, we are increasingly make our research data available and Data will be made available on request.

Declaration
Conflict of interest Authors declare that we have no conflict of interest. We are agreed upon all the Ethical Rules applicable for this journal. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.